Pollution control devices are employed on motor vehicles to control atmospheric pollution. Such devices include a pollution control element. Exemplary pollution control devices include catalytic converters and diesel particulate filters or traps. Catalytic converters typically contain a ceramic monolithic structure having walls that support the catalyst. The catalyst typically oxidizes carbon monoxide and hydrocarbons, and reduces the oxides of nitrogen in engine exhaust gases to control atmospheric pollution. The monolithic structure may also be made of metal. Diesel particulate filters or traps typically include wall flow filters that are often honeycombed monolithic structures made, for example, from porous ceramic materials. The filters typically remove soot and other exhaust particulate from the engine exhaust gases. Each of these devices has a housing (typically made of a metal like stainless steel) that holds the pollution control element. Monolithic pollution control elements, are often described by their wall thickness and the number of openings or cells per square inch (cpsi). In the early 1970s, ceramic monolithic pollution control elements with a wall thickness of 12 mils (304 micrometer) and a cell density of 300 cpsi (47 cells/cm2) were common (“300/12 monoliths”).
As emission laws become more stringent, wall thicknesses have decreased as a way of increasing geometric surface area, decreasing heat capacity and decreasing pressure drop of the monolith. The standard has progressed to 900/2 monoliths. With their thin walls, ceramic monolithic structures are fragile and susceptible to vibration or shock damage and breakage. The damaging forces may come from rough handling or dropping during the assembly of the pollution control device, from engine vibration or from travel over rough roads. The ceramic monoliths are also subject to damage due to high thermal shock, such as from contact with road spray.
The ceramic monoliths have a coefficient of thermal expansion generally an order of magnitude less than the metal housing which contains them. For instance, the gap between the peripheral wall of the metal housing and the monolith may start at about 4 mm, and may increase a total of about 0.33 mm as the engine heats the catalytic converter monolithic element from 25° C. to a maximum operating temperature of about 900° C. At the same time, the metallic housing increases from a temperature of about 25° C. to about 530° C. Even though the metallic housing undergoes a smaller temperature change, the higher coefficient of thermal expansion of the metallic housing causes the housing to expand to a larger peripheral size faster than the expansion of the monolithic element. Such thermal cycling typically occurs hundreds or thousands of times during the life of the vehicle.
To avoid damage to the ceramic monoliths from road shock and vibrations, to compensate for the thermal expansion difference, and to prevent exhaust gases from passing between the monoliths and the metal housings (thereby bypassing the catalyst), mounting mats are disposed between the ceramic monoliths and the metal housings. The process of placing the monolith within the housing is also called canning and includes such steps as wrapping a sheet of mat material around the monolith, inserting the wrapped monolith into the housing, pressing the housing closed, and welding flanges along the lateral edges of the housing.
Typically, the mounting mat materials include inorganic fibers, optionally intumescent materials, organic binders, fillers and other adjuvants. Known mat materials, used for mounting a monolith in a housing are described in, for example, U.S. Pat No. 3,916,057 (Hatch et al.), U.S. Pat. No. 4,305,992 (Langer et al.), U.S. Pat. No. 4,385,135 (Langer et al.), U.S. Pat. No. 5,254,410 (Langer et al.), U.S. Pat. No. 5,242,871 (Hashimoto et al.), U.S. Pat. No. 3,001,571 (Hatch), 5,385,873 (MacNeil), and U.S. Pat. No. 5,207,989 (MacNeil), GB 1,522,646 (Wood) published Aug. 23, 1978, Japanese Kokai No.: J.P. Sho. 58 - 13683 published Jan. 26, 1983 (i.e., Pat Appin Publn No. J.P. Hei. 2 - 43786 and Appin No. J.P. Sho. 56 - 1 12413), and Japanese Kokai No.: J.P. Sho. 56 - 85012 published Jul. 10, 1981 (i.e., Pat. Appln No. Sho. 54-168541). Mounting materials should remain very resilient at a full range of operating temperatures over a prolonged period of use.
A need exists for a mounting system which is sufficiently resilient and compressible to accommodate the changing gap between the monolith and the metal housing over a wide range of operating temperatures and a large number of thermal cycles. While the state of the art mounting materials have their own utilities and advantages, there remains an ongoing need to improve mounting materials for use in pollution control devices. Additionally, one of the primary concerns in forming the mounting mat is balancing between the cost of the materials and performance attributes. It is desirable to provide such a high quality mounting system at the lowest possible cost.
Mounting mats for mounting pollution control devices or monoliths have been produced predominantly by wet laid processes. In particular, wet laid processes are used to produce intumescent mounting mats. The wet laid processes however are expensive as they require substantial investments in equipment and further consume large amounts of energy due to required drying. Additionally, the process typically involves large volumes of aqueous based solutions that need to be handled as well as the associated waste streams, which may need to be treated for environmental reasons. Further, formulating a mounting mat of a particular composition, e.g. having certain desired adjuvants is complicated because of the different interactions of the components of a desired formulation. Moreover, wet laid processes typically require the use of substantial amounts of organic binders to avoid cracking of the mat during mounting. This is particularly so if the mounting mat includes additives such as for example intumescent materials. The use of organic binders is undesirable particularly in mounting mats that are intended for use in ‘low temperature’ catalytic converters, such as with diesel engines where the temperature of the exhaust is typically much lower than with most gasoline engines. Organic binders are also undesirable because of environmental reasons as the organic binders need to be burnt out after assembly of the converter.
Also, the fiber lengths that can be used in a wet laid process may impose limitations.
Dry laid processes have also been used to make mounting mats. For example mounting mats have been produced using commercially available web forming machines such as those marketed under the trade designation “RANDO WEBBER” by Rando Machine Corp. of Macedon, N.Y.; or “DAN WEB” by ScanWeb Co. of Denmark, wherein the fibers are drawn onto a wire screen or mesh belt. Unfortunately, each of these machines comes with its own limitations relative to making mounting mats, thus limiting their usefulness to particular mounting mat formulations optimized for use with these machines. For example, the fiber lengths that can be used on these machines is typically limited. Additionally, adjuvants desired in the formulation of a mounting mat may not be compatible with these machines or their use may lead to mounting mats that do not meet desired performance or may lead to mats with a large variation of performance. Still further, the known dry laid processes may be too aggressive resulting in undesired fiber breakage, irreproducible performance, dust forming in the manufacturing, etc.
Accordingly, the need exists to find a further method for making mounting mats. It would in particular be desirable to find a mat that allows for the manufacturing of a large variety of mounting mats of different formulations including non-intumescent as well as intumescent materials. It would further be desirable to find a method that allows for producing mounting mats at low cost and in a convenient way. It would also be desirable to find a method that can be used to produce mounting mats that have no or a very low amount of binder, in particular mats that are low in binder content and that may include further adjuvants such as for example particles or intumescent materials. Of course, the desired method should typically allow producing the desired mounting mats having a level of performance equal or better than those produced by other methods that have so far been used to produce mounting mats. Typically, the method should allow for making mounting mats of a consistent quality. Satisfactory quality of mounting mats can be achieved, for example, by using inorganic fibers having a low shot content. Therefore, it is also desirable to find a process that reduces the shot content of inorganic fibers suitable for use in mounting mats, in particular dry fibers. Preferably that process can be combined with or integrated in a process of making mounting mats.